An electron tube comprises a vacuum chamber through which an electron beam is propagated from an electron gun to a collector. Such a vacuum chamber is sealed with a sealing structure which comprises a cylindrical insulator of ceramic material having metallized layers formed on sealing portions at both ends thereof, and sealing plates having a coefficient of thermal expansion approximately equal to that of the insulator, wherein the sealing plates are soldered to the metallized layers by use of, for instance, gold solder.
In the conventional sealing structure, however, there is a disadvantage that a creeping discharge occurs on the inner surface of the insulator when a predetermined voltage is applied across the insulator. Electrons are emitted from the metallized layer which is positioned on the side of a negative electrode. The electrons thus emitted collide with the inner surface of the insulator, producing a secondary emission which triggers further emission of electrons flowing from the negative electrode to a corresponding positive electrode.
To avoid such a creeping discharge in the inner surface of the cylindrical insulator, the length thereof is increased to decrease the gradient of a potential applied thereto. Furthermore, either the inner surface of the cylindrical insulator or a metallized layer exposed to the inner surface thereof is polished after the cylindrical insulator is assembled.